Tree composition and abundance patterns differ in two watersheds of Forest Park, a large Pacific Northwest urban forest

Abstract

We compared differences in patterns of tree species composition and abundance in a set of six permanent research plots designated in paired watersheds in a large forested park in Portland, Oregon: one proximal to the urban area and the other proximal to rural areas. All were established in mixed conifer-deciduous stands with similar tree age classes, orientation, topography and other environmental variables. One old-growth plot reference was included in each watershed. The urban plots have experienced periodic disturbances and are continually subjected to continual low-impact disturbances. The urban plots, in contrast to rural plots and to their reference old-growth plots, showed significant differences in tree species composition and ages, sapling numbers, and amount of coarse woody debris. NMDS and nonparametric ANOVA revealed the urban plots as having the greatest ecological distance from all other plots. Soil disturbance, lack of O- and A-horizons, and coarse woody debris were disturbance indicators. Given current patterns of regeneration, the urban plots will be dominated in the future by hardwood species. This projection, when compared with the proximate old-growth patch and with historical records for the urban end of the park, has no historic antecedent. These conclusions are concurrent with findings for other urban forests, where early successional species dominate over long periods of time. The described ecological trends, especially with the domination of invasive species, reflect the urban condition of the forest and constitute a novel state.